Filtration's effect on chlorine demand?

sbcpool

Well-known member
Mar 10, 2015
590
Upland, CA
I installed a new Intelliflo a couple of weeks ago. Coming from a single-speed power hog it's been relatively nice so far. I am taking advantage of the increased efficiency by doing low flow for long periods.

The other day I noticed a change in my pool - I caught a whiff of chlorine while walking by. I got out the test kit and my FC was 10.4!!! Still within SLAM levels, but WAY higher than normal. I haven't changed chlorine brands or dosing. My only theory is that the increased filtration from longer pump run times has dramatically decreased chlorine demand. I am now only losing about 10% per day where I was losing 30 - 33% per day with CYA ~60.

Is my hypothesis plausible, or could it be something else?
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,983
Tucson, AZ
Pool Size
16000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Hmmmmm....wouldn’t have expected that.

Is the SWG gone?

Have you seen any return of mustard algae?
 

sbcpool

Well-known member
Mar 10, 2015
590
Upland, CA
SWG is gone for about 3 weeks now. Even at FC 10 I still see the very slightest, barely detectable yellow on the shaded walls after a couple of weeks. You can't see it when it's on the wall, but when I brush I can see a very faint (barely perceptible) yellow cloud rise in the wake of the brush.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
39,104
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Is it smoky in your area? That really impacts UV on your pool water surface.
 
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mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
39,104
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
That's it. We had a few days of heavy smoke before the last cold front and our pool water temp dropped like that. I put the cover on.
 

sbcpool

Well-known member
Mar 10, 2015
590
Upland, CA
My UV sensor registers a UV index of 2 - 3 for the last week as opposed to 4 - 5 the week before. Makes sense. I never thought of correlating my UV sensor data with FC degradation.
 

sbcpool

Well-known member
Mar 10, 2015
590
Upland, CA
If I average the 5-minute UV index for all hours that sunlight is on the pool it seems that my pool sees about 1% FC loss for each unit of UV index/hour. In other words, if a day sees 30 UV index hours my pool loses about 30% FC. If it's 10 my pool loses about 10% FC.
 
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Oly

Gold Supporter
Jun 28, 2017
2,011
Fresno, CA
Pool Size
27000
Yes I too have seen a fall off of FC usage per day due to reduced solar radiation from smoke. Pool temps also falling.
 
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